1. Field of the Invention
This invention relates to photoconductive elements. More particularly it relates to photoconductive elements which employ a novel barrier layer of aluminum hydroxyoxide crystallites. Such elements are useful in electrophotographic copying processes.
2. Description of the Prior Art
The use of electorphotographic copying has gained widespread acceptance. In this type of reproduction a photoconductive element is first given a uniform electorstatic charge in order to sensitize its photoconductive surface. The element is then imagewise exposed to activating electromagnetic radiation thereby selectivley dissipating the charge in the illuminated areas of the photoconductive element while leaving behind a latent electrostatic image in the non-illuminted areas. This latent electorstatic image may be developed and made visible by, for example, depositing developing material (e.g., finely divided marking particles such as toner particles) on the charged surface of the photoconductive element. If the photoconductive element is of the reusable type, the toner image is then transferred to a second surface (e.g., a sheet of paper) and fixed in place thereon to form a permanent, visible reproduction of the original. If, on the other hand, an inexpensive non-reusable photoconductive element is employed the toner particles may be fixed in place directly on the surface of the element with the consequent eleimination from the process of a transfer step.
Frequently, reusable photoconductive elements comprise an electrically conductive substrate, a barrier layer on one surface therof and a photocaonductive layer on th barrier layer. Barrier layers are employed so as to reduce charge leakage in the absence of activating radiation. This phenomenon, known as dark discharge, brings about premature reduction in the electrostatic charge of image areas thereby reducing the image density on copies produced. It also limits the number of copies that can be produced from a single imaging.
A variety of materials have been suggested as barrier layers . Typically these layers comprise a thin dielectric material which is only a fraction of the thickness of the photoconductive material and is located between the substrate and the photoconductive layer. Such materials include, for example, thin layers or films of aluminum oxide (Al.sub.2 O.sub.3) such as are described in U.S. Pat. No. 2,901,348. However, in order to form a satisfactory Al.sub.2 O.sub.3 layer on an aluminum surface it is necessary that the naturally occuring dense Al.sub.2 O.sub.3 layer be first removed (e.g., by contacting the surface with an acid bath) and then a uniformly thick Al.sub.2 O.sub.3 layer be deposited on the cleaned surface. U.S. Pat. No. 3,940,270 discloses a duplex barrier layer of porous-type Al.sub.2 O.sub.3 and barrier-type Al.sub.2 O.sub.3. The two layers are formed consecutively by electrolytic oxidation. The electorlyte comprises a solution of a strong acid. Potentials of up to 500 volts are used during oxidation.
The adhesion of photoconductive materials to such barrier layers can be marginal. Thus, it is frequently necessary to "pair" a barrier with a particular photoconductive layer so as to obtain adequate adhesion of the latter to the former. Alternatively, the use of adhesion-promoting layers has been suggested.
For example, aluminum hydroxyoxide has been suggested so as to bond particulate material to aluminum substrates in U.S. Pat. Nos. 3,871,881 and 3,975,197. These patents describe the depostion of particulate material upon the substrate with the subsequent in-situ formation of aluminum hydroxyoxide crystals around the particles.
Netherlands Patent Publication No. 7,410,265 describes the use of a sealed anodically formed porous aluminum oxide coating between an aluminum substrate and a photoconductive layer of selenium in order to enhance the adhesion of the photoconductor to the substrate. In the process the substrate is first preferably cleaned. The naturally occurring non-porous Al.sub.2 O.sub.3 layer is then removed. The substrate is then electrically anodized to form a uniform layer of porous Al.sub.2 O.sub.3. This layer is then contacted with conditions and chemicals which hydrate Al.sub.2 O.sub.3 sufficiently to seal the pores thereof.
While these types of constructions have met with some success, they have not proven entirely satisfactory. For example the techniques of preparing such constructions have several disadvantages attendant therewith. Certain of these procedures require the use of highly acidic materials in order to remove the Al.sub.2 O.sub.3. Others require special baths and techniques in order to anodize the aluminum surface. In addtion to being time consuming and expensive, such processes also give rise to water pollution problems.
These and other disadvantages of the prior art have been overcome by the present invention by the use of a barrier layer of aluminum hydroxyoxide crystallites in photoconductive elements. Elements of the present invention eliminate the need to employ layers of particulate photoconductive material or special techniques to remove the naturally occurring aluminum oxide layer. Moreover, elements of the invention require neither the depostion of special aluminum oxide layers, nor the anodization of their aluminum surface.
The elements of the invention exhibit excellent dark decay characteristics, excellent charge uniformity and good resistance to charge decay. Futhermore, the barrier layers of elements of the invention have outstanding adhesion to both the substrate and the overlying photoconductive layer. Moreover, the preparation of the barrier layers is accomplished by a quick and simple process which is inexpensive and pollution free.